1. Field of the Invention
The present invention relates to a switch device, and in particular, to a structure of a switch device used in a temperature controller of electric cooking appliance such as an electric oven.
2. Description of the Related Art
As conventional switch devices, a switch device is known in which the device is actuated by a control group comprising a temperature sensor such as a heating type thermo-bimetal in controlling the output of a heater of an electric oven (See Japanese Patent No. 2631376).
FIGS. 12 and 13 show a structure of a conventional switch device. FIG. 12 is a plan view showing the inside of the switch device, and FIG. 13 is a partially detailed view showing principal parts of the device.
In these drawings, a switch device 101 is a time-setting-type output control device comprising an appliance switch 106. This switch 106 is actuated by a heating-type thermo-bimetal 115. The switch device 101 is a snap switch. The switch device 101 has an adjusting unit 139 for changing the gap between a contact 136 and a counter stop 137. The counter stop 137 is used for a movable switch contact 134 and is actuated for changing the temperature setting range of the switch device.
In other words, in a lower setting range, the switching hysteresis of the appliance switch 106 becomes smaller, and in a higher setting range, the switching hysteresis of the appliance switch 106 becomes larger. Therefore, in the appliance switch 106, the time of connection during which the switch is turned on can be precisely set to a smaller value. As a result, in the lower setting range, the output control can be very precisely performed.
The switch device 101 is actuated in the following way. In a switched-off position, the switch contact 134 faces the contact 136, but making no contact, and is at a stop position of the counter stop 137. When a setting spindle 108 is located at a position in a switched-on range, the switch contact 134 moves to its closed position. In this closed position, the switch contact 134 abuts on the contact 136 by the levering action that is caused between a switch element 114 and a runner 146. A heating resistor 116 is switched on aside from electric appliance to be controlled.
When the heating resistor 116 heats the thermo-bimetal 115, the switch element 114 is biased toward the side on which a compensating unit 120 abuts on a spiral spring 118. In other words, the switch element 114 is biased in the direction that abuts on a pressing plate of a snap spring. This operation continues until a bi-stable snap spring 131 reaches its dead point, i.e., its switching point, and the snap spring 131 is resiliently bounced in the direction of the position of the other end thereof.
As a result, the switch contact 134 is separated from the contact 137 and abuts on the counter stop 137. Since the switch contact 134 is opened, current does not flow through the heating resistor 116. Then, the thermo-bimetal 115 returns back to the cold initial position. In other words, the dead point of the snap spring 131 is returned to its original position, and the snap spring 131 again is resiliently bounced toward the position of one end thereof. As a result, the switch contact 134 returns to its closed position.
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
However, in a construction of the above-described conventional switch device, a switch contact is operated by the balance between a force from the bimetal and a force from the snap spring, the operational speed of the bimetal is slow. Therefore, arcs tend to fly at the time of the switching of the connection between the switching contacts. Further, since the force that pushes the switching contacts becomes small, the bouncing of the contacts tends to appear.